Add the `cfg_match!` macro
# Movitation
Adds a match-like version of the `cfg_if` crate without a RFC [for the same reasons that caused `matches!` to be included in the standard library](https://github.com/rust-lang/rust/pull/65479).
* General-purpose (not domain-specific)
* Simple (the implementation is short) and useful (things can become difficult with several `cfg`s)
* Very popular [on crates.io ](https://crates.io/crates/cfg-if) (currently 3th in all-time downloads)
* The two previous points combined make it number three in [left-pad index](https://twitter.com/bascule/status/1184523027888988160) score
```rust
match_cfg! {
cfg(unix) => {
fn foo() { /* unix specific functionality */ }
}
cfg(target_pointer_width = "32") => {
fn foo() { /* non-unix, 32-bit functionality */ }
}
_ => {
fn foo() { /* fallback implementation */ }
}
}
```
# Considerations
A match-like syntax feels more natural in the sense that each macro fragment resembles an arm but I personally don't mind switching to any other desired syntax.
The lack of `#[ ... ]` is intended to reduce typing, nevertheless, the same reasoning described above can also be applied to this aspect.
Since blocks are intended to only contain items, anything but `cfg` is not expected to be supported at the current or future time.
~~Credits goes to `@gnzlbg` because most of the code was shamelessly copied from https://github.com/gnzlbg/match_cfg.~~
Credits goes to `@alexcrichton` because most of the code was shamelessly copied from https://github.com/rust-lang/cfg-if.
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